Low-emittance Beam Research Articles

Present status and first results of the final focus beam line at the KEK Accelerator Test Facility

ATF2 is a final-focus test beam line which aims to focus the low emittance beam from the ATF damping ring to a vertical size of about 37 nm and to demonstrate nanometer level beam stability. Several advanced beam diagnostics and feedback tools are used. In December 2008, construction and installation were completed and beam commissioning started, supported by an international team of Asian, European, and U.S. scientists. The present status and first results are described.Received 1 November 2009DOI:https://doi.org/10.1103/PhysRevSTAB.13.042801This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Development of new muon source for muon g-2 measurement and muon magnetic microscope

In various fields, from particle physics to condensed matter physics, extremely low emittance muon beam is eagerly awaited. We started a new project to realize the extremely low emittance and high intensity muon beam further beyond previous R&D study performed at RIKEN-RAL. To reduce the muon beam emittance, we plan to utilize the room temperature muonium generator, and furthermore, we apply muonium motion-cancellation mechanism using Doppler shift of 1S-2P transition energy. By the cancellation, one can expect the beam emittance better than ΔxΔθx ≈ 100 μm·mrad even at 100 kV extraction field, which can be focused to the size of ~ 1μm by applying electron microscope technique. We are also developing high intensity and extremely stable VUV laser for the muonium ionization, which can efficiently ionize muonium and produce high muon yield of ~ 10 kHz even at RIKEN-RAL (240kW proton with conventional surface muon channel). This new R&D study will open a totally new application to the muon science, namely a Muon Magnetic Microscope (M3). This muon beam enable us to observe sub-μmeter magnetic structure of the condensed matter with the depth resolution less than 10 nano-meter.

Possible operation of the European XFEL with ultra-low emittance beams

Recent successful lasing of the Linac Coherent Light Source (LCLS) in the hard X-ray regime and the experimental demonstration of a possibility to produce low-charge bunches with ultra-small normalized emittance have lead to the discussions on optimistic scenarios of operation of the European XFEL. In this paper we consider new options that make use of low-emittance beams, a relatively high beam energy, tunable-gap undulators, and a multi-bunch capability of this facility. We study the possibility of operation of a spontaneous radiator (combining two of them, U1 and U2, in one beamline) in the SASE mode in the designed photon energy range 20–90 keV and show that it becomes possible with ultra-low emittance electron beams similar to those generated in LCLS. As an additional attractive option we consider the generation of powerful soft X-ray and VUV radiation by the same electron bunch for pump-probe experiments, making use of recently invented compact afterburner scheme. We also propose a betatron switcher as a simple, cheap, and robust solution for multi-color operation of SASE1 and SASE2 undulators, allowing to generate 2–5 X-ray beams of different independent colors from each of these undulators for simultaneous multi-user operation. We describe a scheme for pump-probe experiments, based on a production of two different colors by two closely spaced electron bunches (produced in photoinjector) with the help of a very fast betatron switcher. Finally, we discuss how without significant modifications of the layout the European XFEL can become a unique facility that continuously covers with powerful, coherent radiation a part of the electromagnetic spectrum from far infrared to gamma-rays.

S-band linac-based X-ray source with π/2-mode electron linac

The activities with the compact X-ray source are attracting more attention, particularly for the applications of the source in medical fields. We propose the fabrication of a compact X-ray source using the SAMEER electron linear accelerator and the KEK laser undulator X-ray source (LUCX) technologies. The linac developed at SAMEER is a standing wave side-coupled S-band linac operating in the π/2 mode. In the proposed system, a photocathode RF gun will inject bunches of electrons in the linac to accelerate and achieve a high-energy, low-emittance beam. This beam will then interact with the laser in the laser cavity to produce X-rays of a type well suited for various applications. The side-coupled structure will make the system more compact, and the π/2 mode of operation will enable a high repetition rate operation, which will help to increase the X-ray yield.

Simulation studies for ion beam extraction systems

The characteristics of the ion beam extracted from an ion sources were investigated using computer code SIMION 3 D Version 7.0. It has been used to evaluate the extraction system in order to produce an ion beam with high current and low beam emittance. The results show that the shape of the extraction electrode plays an important role in ion beam formation. Comparison has been made between two extraction systems, Pierce extraction electrode and spherical extraction electrode. The results show that the spherical extraction system yields ion extraction beam with lower emittance and radius than that the Pierce system. The simulation can provide the basis for optimizing the extraction system and the acceleration gap for ion source.

Use of particle reflection in curved crystals to improve beam collimation in a ring accelerator

The staff of the Institute of High-Energy Physics together with researchers from Russian and foreign scientific centers have discovered a new physical phenomenon – reflection of fast charge particles in curved crystals. This article presents the first experimental results using multiple reflections of protons in silicon crystals to increase the beam collimation efficiency in the U-70 accelerator. The experimental data are compared with theoretical models. The technology developed using reflecting crystals can be used to form oppositely propagating beams in accelerators and has prospects for extracting low-emittance beams on ~1 GeV accelerators for electronuclear facilities and in medicine and industry.

Longitudinal Laser Pulse Shaping for Low-Emittance Electron-Beam Generation

Longitudinal Laser Pulse Shaping for Low-Emittance Electron-Beam Generation Kiho Hyun, Chang-Mook Yim, Yong Woon Parc, Jang Ho Park and In Soo Ko Department of Physics, Pohang University of Science and Technology, Pohang 790-784 Changbum Kim and Jaehun Park Pohang Accelerator Laboratory, Pohang University of Science and Technology, Pohang 790-784 (Received 21 August 2007, in nal form 30 August 2008) Several X-ray free electron laser (FEL) projects require a very low-emittance electron beam and laser pulse shaping is needed for low-emittance electron-beam generation. Especially, a uniform laser pulse shape is very e cient. We have designed a 4-pulse generator for longitudinally uniform laser pulse shaping and a cross-correlation system to diagnose the shaped pulse. In this article, we introduce a 4-pulse generator for longitudinal laser pulse shaping and a technique to make a longitudinally uniform shape. In addition, we introduce a specially designed cross-correlation system for mixed polarizations, with which we can diagnose the laser pulse. PACS numbers: 29.25.Bx

A compact high-resolution isobar separator for the CARIBU project

A compact high-resolution isobar separator for the CARIBU project at Argonne National Laboratory has been designed. It has a first-order mass resolution of 22,400:1, and a >95% transmission for the 50keV low-emittance beam from a gas catcher–cooler combination.

A high-current electron beam ion trap as a charge breeder for the reacceleration of rare isotopes at the NSCL (invited)

Reacceleration of low-energy rare isotope beams available from gas stopping of fast-fragment beams or from an ISOL target station to energies in the range of 0.3-12 MeV/nucleon is needed for experiments such as low-energy Coulomb excitation and transfer reaction studies and for the precise study of astrophysical reactions. The implementation of charge breeding as a first step in a reaccelerator is a key to obtaining a compact and cost-efficient reacceleration scheme. For highest efficiency it is essential that single charge states are obtained in a short breeding time. A low-emittance beam must be delivered. An electron beam ion trap (EBIT) has the potential to meet these requirements. An EBIT-based charge breeder is presently under design and construction at the NSCL as part of the construction of a reaccelerator for stopped beams from projectile fragmentation. This new facility will have the potential to provide low-energy rare isotope beams not yet available elsewhere.

Progress report on the LCLS XFEL at SLAC

The Linac Coherent Light Source (LCLS) Project will be an x-ray free-electron laser. It is intended to produce pulses of 800-8,000 eV photons. Each pulse, produced with a repetition frequency of up to 120 Hz, will provide >1012 photons within a duration of less than 200 femtoseconds. The project employs the last kilometer of the SLAC linac to provide a low-emittance electron beam in the energy range 4-14 GeV to a single undulator. Two experiment halls, located 100 m and 350 m from the undulator exit, will house six experiment stations for research in atomic/molecular physics, pump-probe dynamics of materials and chemical processes, x-ray imaging of clusters and complex molecules, and plasma physics. Engineering design activities began in 2003, and the project is to be completed in the middle of 2010. The project design permits straightforward expansion of the LCLS to multiple undulators.

Thermal emittance measurements for electron beams produced from bulk and superlattice negative electron affinity photocathodes

Extremely low emittance electron beams are required for next generation accelerators. GaAs semiconductor photocathodes with negative electron affinity (NEA) surfaces have an intrinsic advantage for generating such low emittance beams and the thermal emittance as low as 0.1 π mm mrad is expected in ideal case. The thermal emittance of photoelectrons was measured for two different NEA photocathodes: a bulk-GaAs photocathode and a GaAs-GaAsP superlattice strained photocathode. The normalized root-mean-sqare emittances for the beam radius of 1.0 mm were as low as 0.20−0.29±0.02 and 0.15±0.02 π mm mrad, respectively. A comparison of these results shows that the superlattice photocathode minimizes the thermal emittance for photon excitation energies higher than the band gap energy.

Quadruple-bend achromatic low emittance lattice studies

A quadruple-bend achromatic (QBA) cell, defined as a supercell made of two double-bend cells with different outer and inner dipole bend angles, is found to provide a factor of 2 in lowering the beam emittance relative to the more conventional double-bend achromat. The ratio of bending angles of the inner dipoles to that of the outer dipoles is numerically found to be about 1.5-1.6 for an optimal low beam emittance in the isomagnetic condition. The QBA lattice provides an advantage over the double-bend achromat or the double-bend nonachromat in performance by providing a small natural beam emittance and some zero-dispersion straight sections. A lattice with 12 QBA cells and a preliminary dynamic aperture study serves as an example.

Field emission dark current of technical metallic electrodes

In the framework of the Low Emittance Gun (LEG) project, high gradient acceleration of a low emittance electron beam will be necessary. In order to achieve this acceleration, a −500 kV, 250 ns FWHM, pulse will be applied between two electrodes. Those electrodes should sustain the pulsed field without arcing, must not outgas and must not emit electrons. Ion back bombardment, and dark current will be damaging to the electron source as well as for the low emittance beam. Electrodes of commercially available OFE copper, aluminium, stainless steel (SS), titanium and molybdenum were tested, following different procedures including plasma glow discharge cleaning.

Field Emission of Spin-Polarized Electrons Extracted from Photoexcited GaAs Tip

A pyramidal-shaped GaAs (tip-GaAs) photocathode for a polarized electron source (PES) was developed to improve beam brightness and negative electron affinity (NEA) lifetime by field emission. The emission mechanism also enables the photocathode to extract electrons from the positive electron affinity (PEA) surface into vacuum, and alleviates the NEA lifetime problem. The measured electrical characteristics of tip-GaAs and its polarization exhibited distinctive field-emission behavior. The polarization of the electron beam extracted from tip-GaAs was 20–38% under irradiation with circularly polarized light of 700–860 nm, and the peak polarization was 37.4±1.4% at a wavelength of 731 nm. These experimental results indicate that spin-polarized electrons can be extracted from the conduction band into vacuum by a field-emission mechanism. This, in turn, shows that this type of photocathode has the prospect of generating a low-emittance spin-polarized electron beam.

On the production of flat electron bunches for laser wakefield acceleration

We suggest a novel method for the injection of electrons into the acceleration phase of particle accelerators, producing low-emittance beams appropriate even for the demanding high-energy linear collider specifications. We discuss the injection mechanism into the acceleration phase of the wakefield in a plasma behind a high-intensity laser pulse, which takes advantage of the laser polarization and focusing. The scheme uses the structurally stable regime of transverse wakewave breaking, when the electron trajectory self-intersection leads to the formation of a flat electron bunch. As shown in three-dimensional particle-in-cell simulations of the interaction of a laser pulse elongated in the transverse direction with an underdense plasma, the electrons injected via the transverse wakewave breaking and accelerated by the wakewave perform betatron oscillations with different amplitudes and frequencies along the two transverse coordinates. The polarization and focusing geometry lead to a way to produce relativistic electron bunches with an asymmetric emittance (flat beam). An approach for generating flat laser-accelerated ion beams is briefly discussed.

Using a deformed crystal for bending a sub-GeV positron beam

We analyze the scattering of 480MeV positrons in bent crystal lattices carried out at the Beam Test Facility of the INFN – Laboratori Nazionali di Frascati. We observe experimentally that some particles follow the bending of the crystal lattice, presumably being guided by channeling phenomenon, and are deflected through the angles of about 10mrad over length of 1mm of silicon. This technique may lead to the use of channeling effect for steering of particle beams at the energies under 1GeV, aimed at the production of ultrastable beams of low emittance for medical and biological applications.

High current superconducting gun at 703.75 MHz

A half-cell superconducting RF (SRF) electron gun has been proposed as an injector to the 20 MeV energy recovery linac (ERL) prototype at Brookhaven National Lab (BNL). The design and optimization of the half-cell gun based on RF parameters, higher order mode (HOM) wakefields, and preservation of very low beam emittance in the high current regime are discussed. Comparison of several different shapes based on the above criteria and issues relating to multipacting, cathode insertion, and laser stability will be presented.

Coherent X-rays at MAMI

Coherent radiation in the range from soft X-rays up to hard X-rays, produced by the low-emittance electron beam of MAMI, can be used for various applications. Novel types of interferometers have been developed for the measurement of the complex index of refraction of thin self-supporting foils. For the vacuum ultraviolet and soft X-ray region the interferometer consists of two collinear undulators, and a grating spectrometer. A foil placed between the undulators causes a phase shift and an attenuation of the oscillation amplitude. The complex index of refraction has been measured at the L 2,3-absorption edges of nickel. A novel method is described for the measurement of the X-ray magnetic circular birefringence. For the hard X-ray region the interferometer consists of two foils at which the 855 MeV electron beam produces transition radiation. Distinct interference oscillations have been observed as a function of both, the photon emission angle and the distance between the foils. The refractive index decrement δ(ω) of a 2 µm thick nickel sample foil has been measured at X-ray energies around the K absorption edge at 8333 eV and at 9930 eV with an accuracy of better than 1.5 %. The line width of parametric X radiation (PXR) was measured in backward geometry with a Si single-crystal monochromator. Upper limits of the line width of 42meV, 50meV, and 44meV, have been determined for the (333), (444) and (555) reflections at photon energies of 5932 eV, 7909 eV, and 9887 eV, respectively. Small angle scattering of the electrons in the crystal leads to a stochastic frequency modulation of the exponentially damped wave train which results in the line broadening. To elucidate the quest if the production of PXR is a kinematical or a dynamical process the radiation from silicon single-crystal targets, emitted close to the electron direction, has been studied. The observed interference structures and the narrow-band radiation in forward direction shows that PXR is produced in a dynamical process.

Steering a multi-MeV positron beam with a curved crystal

We observe positron bending by a crystal lattice, presumably being guided by a channeling phenomenon, deflecting the beam by about 10 milliradian over a length of 1 mm of silicon. This technique may lead to the use of the channeling effect for steering particle beams at energies below 1 GeV, for the purpose of producing beams of low emittance with enhanced stability for medical and biological applications.

Beam diagnostics for high quality electron beam emitted from photocathode rf-gun

High quality electron beam generation using photocathode rf-gun system and beam diagnostic techniques have been developed at Waseda University. This system can generate up to 4.6 MeV low emittance and short bunch electron beam. For bunch length monitor, rf-kicker technique is able to streak the electron bunch directly, using transverse magnetic field on the beam orbit. This technique is able to observe the longitudinal profile of the bunch and to achieve higher resolution than streak camera method.